Title:Neutral sodium from comet Hale-Bopp: a third type of tail

Abstract: We report on the discovery and analysis of a striking neutral sodium gas tail
associated with comet C/1995 O1 Hale-Bopp. Sodium D-line emission has been
observed at heliocentric distance r<1.4 AU in some long-period comets and the
presence of neutral sodium in the tailward direction of a few bright comets has
been noted, but the extent, and in particular the source, has never been clear.
Here we describe the first observations and analysis of a neutral sodium gas
tail in comet Hale-Bopp, entirely different from the previously known ion and
dust tails. We show that the observed characteristics of this third type of
tail are consistent with itbeing produced by radiation pressure due to
resonance fluorescence of sodium atoms and that the lifetime for
photoionization is consistent with recent theoretical calculation.